High-voltage metal-oxide semiconductor

ABSTRACT

A high voltage metal oxide semiconductor device. The high voltage device comprises a high voltage NMOS, a high voltage PMOS, or a high voltage CMOS. A field oxide layer is used to isolate the gate from the source region, while a diffusion region is formed under the field oxide layer. A channel region around the source drain extends across a first doped well and a second doped well having different dopant concentration. The channel region further comprises two grading regions with different dopant concentrations around the drain region.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a semiconductor fabricating method. More particularly, the present invention relates to a method of forming a metal oxide semiconductor.

2. Description of Related Art

Due to the increasing number of semiconductor elements incorporated in integrated circuits, the size of metal oxide semiconductor (MOS) components is greatly reduced. Accordingly, as the channel length of the MOS is decreased, the operating speed is increased. However, there is an increased likelihood of a problem, referred to as “short channel effect”, caused by the reduced channel length. If the voltage level is fixed, according to the equation of “electrical field=electrical voltage/channel length”, as the channel length is shortened, the strength of electrical field is increased. Thus, as the intensity of electrical filed increases, electrical activity increases and electrical breakdown is likely to occur.

To solve the problem of electrical breakdown, a method to fabricate a high voltage device being able to withstand a high intensity of electric field has been developed. An isolation structure and a drift region, which is below the isolation structure, are formed on a substrate between a gate and a source/drain of a MOS to increase the distance between the source/drain region and the gate.

In the application of radio frequency (RF), a higher power gain is required to improve frequency response. The method to obtain a higher power gain is to increase the transconductance of the devices. While increasing the transconductance of devices, the intensity of electrical field of the junction between the source region and the channel region increases. In other words, as the electrical field of the channel region increases, the transconductance of the device is increased. In order to avoid the short channel effect and electrical breakdown, the electrical field of channel region must be limited. Thus, a high transconductance is difficult to obtain in the conventional fabrication method of a MOS.

FIG. 1 is a cross-sectional view showing a conventional fabrication process of forming a lateral double-diffused MOS (LDMOS).

In FIG. 1, a conventional LDMOS includes a P-type substrate 100, a field oxide layer 101, a gate oxide layer 102, a gate layer 103, an N⁺ drain region 104, an N⁻ drift region 105, a N⁺ source region 106, and a P-doped region 107.

The dopant concentration in the N⁻ drift region 105 is lightened in the conventional LDMOS in order to achieve a high voltage operation. However, this level of enhance voltage is limited, and consequently, the driving current is reduced. In the application of radio frequency, a higher transconductance is required, that is, the intensity of electrical field strength at the junction between the source region and the channel is increased, or the dopant concentration in the P-doped region of the source region is increased. In this manner, an electrical breakdown is easily caused. Hence, the increase of transconductance is not easy to achieve. Therefore, the application of conventional LDMOS is limited.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a fabricating method of a high-voltage MOS device. The transconductance of the device is increased to withstand a high breakdown voltage with an enhanced current driving performance.

It is another object of the invention to provide a method of forming a high-voltage MOS device to form a channel region comprising a first part being heavily doped and a second part being lightly doped. The first part is applied with an electrical field with a high intensity to increase the transconductance, while the second part prevents electric breakdown.

It is yet another object of the invnetion to provide a method to fabricate a high voltage MOS device. Two portions with different doping concentration are formed aside the drain region of the MOS device. Apart from preventing electric breakdown as mentioned above, the performance of driving current is enhanced.

Accordingly, the present invention provides a fabricating method of a high-voltage metal oxide semiconductor. Two channel regions with different concentrations are formed in a channel region. Two grading regions with different concentrations are formed around the side wall of drain region. The channel region with high concentration can increase the internal electrical field. And thus, the transconductance of components is increased. The other channel region with low concentration can be used to avoid the electrical breakdown. Moreover, the two grading regions formed around the sidewall of the drain region not only avoid electrical breakdown but also increases the capability of current driving.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,

FIG. 1 is a cross-sectional view showing a structure of a conventional LDMOS; and

FIGS. 2A through 2G are cross-sectional views of a portion of a semiconductor showing the steps of fabricating a high-voltage LDMOS according to one preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

The fabricating process of a high-voltage MOS is shown in the following FIGS. 2A through 2G according to one preferred embodiment of the invention. A P-type region 200 a and an N-type region 200 b are provided. The combination of the P-type region 200 a and the N-type region 220 b may be that P-type region 220 a is included in the N-type region 220 b, or on the contrary, the N-type region 220 b is included in the P-type region 220 a. Or alternatively, both the P-type and the N-type region 220 a and 220 b are formed as a twin well structure in a single substrate, or even a P-type epitaxy layer and an N-type epitaxy layer on an insulation substrate, respectively. In this embodiment, the P-type region 220 a and the N-type region 220 b are separately sketched as shown in the FIG. 2A to FIG. 2G in order to avoid the restriction of the application of this invention. The fabricating process may be applied to a MOS device, such as NMOS or a PNOS, and a complementary MOS (CMOS).

In FIG. 2A, an oxide layer 202 a and 202 b is formed to cover the P-type region 200 a and the N-type region 200 b, respectively. A P-type ion implantation is performed to form a P-well 203 a in the P-type region 200 a and a P-well 203 b in the N-type region 200 b.

In FIG. 2B, an N-type ion implantation is performed to form an N-well 204 a in the P-type region 200 a and an N-well in the N-type region 200 b. An ion drive-in step is performed.

In FIG. 2C, a second P-type ion implantation is performed to form a P-well 205 a in the P-well 203 a and a P-well 205 b in the P-well 203 b.

In FIG. 2D, another N-type ion implantation is performed to form a N-well 206 a in the N-well 204 a and a P-well 206 b in the P-well 204 b.

In FIG. 2E, the oxide layer 202 a and 202 b is removed. A pad oxide layer 213 a and 213 b is formed on the P-type regions 200 a and 200 b. A nitride silicon layer 207 a and 207 b is formed on the pad oxide layer 213 a and 213 b. Openings 214 a and 214 b are formed in the nitride silicon layer 207 a and 207 b to expose parts of the P-type region 200 a and N-type region 200 b. The exposed part of the P-type region 220 a includes an area across the N-well 204 a, the N-well 206 a and a part of the bulk surface of the P-region 200 a. The exposed part of the N-type region 220 b includes an area across the P-well 203 b, the P-well 205 b, and a part of the bulk surface of the N-type region 200 b. An N⁻ ion implantation is performed to form an N⁻ drift region 208 a in the substrate 200 a. A P⁻ ion implantation is performed to form a P⁻ drift region 208 b in the substrate 200 b. A thermal oxidation step is performed to form a filed oxide layer 209 a on the N⁻ drift region 208 a and a filed oxide layer 209 b on a P⁻ drift region 208 b.

In FIG. 2F. the silicon nitride layer 207 a and 207 b and the pad oxide layer 213 a and 213 b are removed. A gate oxide layer 201 a and 201 b is formed on the substrate 200 a and 200 b. A polysilicon layer (not shown) is formed over the substrate 200 a and 200 b. The polysilicon layer is patterned to form a gate layer 210 a and 210 b on the gate oxide layer. The gate layer 210 a covers the gate oxide layer 201 a over a part of the P-wells 203 a and 205 a, and a part of the field oxide layer 209 a. Whereas, the gate layer 210 b covers a part of the N-wells 204 b and 206 b and the field oxide layer 209 b. The polysilicon layer is for example, a doped polysilicon layer.

In FIG. 2G, a N⁺ source/drain ion implantation is performed to form an N⁺ drain region 211 a in the N-well 206 a and an N⁺ source region 212 a in the P-well 205 a. A P⁺ source/drain ion implantation is performed to form a P⁺ drain region 211 b in the P-well 205 b and a P⁺ source region 212 b in the N-well 206 b. An annealing step is performed, and a LDMOS device is formed.

As shown in FIG. 2G, the high-voltage LDNMOS structure is formed on a substrate 200 a. A gate oxide layer 201 a is formed on the P-type region 200 a. The LDNMOS comprises a gate layer 210 a on the gate oxide layer 201 a, a N⁺ drain region 211 a and an N⁺ source region 212 a. A field oxide layer 209 a is formed between the gate layer 210 a and the N⁺ drain region 211 a. A P⁻ drift region 208 a is formed under the field oxide layer 209 a. The N source region 212 a is encompassed by the P-well 205 a, while the P-well 205 a is encompassed by the P-well 203 a. Similarly, the N⁺ drain region 211 a is encompassed by the N-well 206 a, while the N-well 206 a is encompassed by the N-well 204 a.

The dopant concentration is in the order of: “the N⁺ drain region 211 a>the N-well 206 a>the N-well 204 a”, and “the P⁻well 205 a>the P-well 203 a>the P-type region 200 a”.

In constrast, the high-voltage LDPMOS structure is formed on an N-type region 200 b. A gate oxide layer 201 b is formed on the N-type region 200 b. The LDPMOS comprises a gate layer 210 b formed on the gate oxide layer 201 b, a P⁺ drain region 211 b and a P⁺ source region 212 b. A field oxide layer 209 b is formed between the gate layer 210 b and the P⁺ drain region 211 b. An N⁻ drift region 208 b is formed under the field oxide layer 209 b. The P⁺ source region 212 b is encompassed by the N-well 206 b, while the P-well 206 b is encompassed by the P-well 204 b. Similarly, the P⁺ drain region 211 b is encompassed by the P-well 205 b, while the P-well 205 b is encompassed by the N-well 204 a.

The dopant concentration is in the order of: “the P⁺ drain region 211 b>the P-well 205 b>the P-well 203 a”, and “the N⁻well 206 b>the P-well 204 a>the N-type region 200 b”.

In the LDNMOS, the channel region under the gate layer 210 a around the N⁺ source region 212 a includes regions across the P-well 205 a and the P-well 203 a. Around the N⁺ drain region 211 a, the channel region further comprises two grading regions, that is, portions of the first N-well 204 a and the N-well 206 a. In contrast, in the LDPMOS, the channel region under the gate layer 210 b around the P⁺ source region includes regions across the second N-well 204 b and the fourth N-well 206 b. Around the P⁻ source region 211 b. the channel region further comprises two grading regions formed of portions of the second P-well 203 b and the fourth P-well 205 b.

As the region of the third P-well region 205 a has a dopant concentration higher than that of the region of the portion of the first P-well region 203 a, the internal electric field is enhanced to obtain a high transconductance. On the other hand, with the formation of the first P-well region 203 a, the N⁺ source region 212 a can thus withstand a high voltage of electric breakdown. The formation of the grading regions may as well increase the breakdown voltage of the N⁺ drain region 211 a, in addition, the driving current performance may also be enhanced. It is apparent that the LDPMOS has a similar structure to the LDNMOS, so that similar effects and advantages may be achieved as the LDNMOS.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents. 

What is claimed is:
 1. A high-voltage metal oxide semiconductor device, comprising: a P-type region; a gate on the P-type region; an N-type source region and an N-type drain region in the P-type region, aside of the gate; a field oxide layer, under a part of the gate to isolate the gate and the N-drain region; an N-type drift region, under the field oxide layer; a first P-well, encompassing the N-type source region in the P-type region; a second P-well, encompassing the first P-well in the P-type region; a first N-well, encompassing the N-type drain region in the P-type region; and a second N-well, encompassing the first N-well in the P-type region; wherein the first P-well has a dopant concentration higher than that of the second P-well.
 2. The device of claim 1, wherein the P-type region comprises a P-type well in a substrate.
 3. The device of claim 1, wherein the P-type region comprises a P-type substrate.
 4. The device of claim 1, wherein the dopant concentration of the second P-well is higher than the dopant concentration of the P-type region.
 5. The device of claim 1, wherein the N-type drain region has a dopant concentration higher than that of the first N-well.
 6. The device of claim 1, wherein the dopant concentration of the first N-well is higher than that of the second N-well.
 7. A high-voltage metal oxide semiconductor device, comprising: an N-type region; a gate on the N-type region; a P-type source region and a P-type drain region in the N-type region, aside of the gate; a field oxide layer, under a part of the gate to isolate the gate and the P-type drain region; an P-type drift region, under the field oxide layer; a first N-well, encompassing the P-type source region in the N-type region; a second N-well, encompassing the first N-well in the N-type region; a first P-well, encompassing the P-type drain region in the N-type region; and a second P-well, encompassing the first P-well in the N-type region; wherein the first N-well has a dopant concentration higher than that of the second N-well.
 8. The device of claim 7, wherein the N-type region comprises a N-type well in a substrate.
 9. The device of claim 7, wherein the N-type region comprises a N-type substrate.
 10. The device of claim 7, wherein the concentration of the second N-well is higher than the dopant concentration of the N-type region.
 11. The device of claim 7, wherein the P-type drain region has a dopant concentration higher than the dopant concentration of the first P-well.
 12. The device of claim 7, wherein the dopant concentration of the first P-well is higher than that of the second P-well.
 13. A high-voltage metal oxide semiconductor device, comprising: a P-type region and an N-type region; a first gate on the P-type region, and a second gate on the N-type region; an N-type source region and an N-type drain region in the P-type region, aside of the first gate, and a P-type source region and a P-type drain region on the N-type region, aside the second gate; a first field oxide layer, under a part of the first gate to isolate the gate and the N-drain region, and a second field oxide layer, under a part of the second gate to isolate the second gate and the P-drain region; a first and second N-type drift region, under the first and the second field oxide layers, respectively; a first P-well, encompassing the N-type source region in the P-type region, and a first N-well, encompassing the P-type source region in the N-type region; a second P-well, encompassing the first P-well in the P-type region, and a second N-well, encompassing the first N-well in the N-type region; a third N-well, encompassing the N-type drain region in the P-type region, and a third P-well, encompassing the P-type drain region in the N-type region; and a fourth N-well, encompassing the first N-well in the P-type region, and a fourth N-well, encompassing the first N-well in the P-type region; wherein the first P-well has a dopant concentration higher than that of the second P-well; and the first N-well has a dopant concentration higher than that of the second N-well.
 14. The device of claim 13, wherein the P-type region comprises a P-type substrate including the N-type region.
 15. The device of claim 13, wherein the N-type region comprises an N-type substrate including the P-type region.
 16. The device of claim 13, wherein the N-type region and the P-type region comprise an N-well and a P-well formed on a substrate, respectively.
 17. The device of claim 13, wherein the N-type region and the P-type comprises an N-type epitaxy and a P-type epitaxy formed on a substrate, respectively.
 18. The device of claim 13, wherein the dopant concentration of the second P-well is higher than that of the P-type region, and the dopant concentration of the second N-well is higher than that of the N-type region.
 19. The device of claim 13, wherein the third N-well has a dopant concentration higher than that of the fourth N-well, and less than that of the N-type drain region.
 20. The device of claim 13, wherein the third P-well has a dopant concentration higher than that of the fourth P-well, and less than that of the P-type drain region. 